Answer: D. can say that Chan is buying the utility-maximizing amounts of donuts and cheese.
Explanation: Utility is the amount of satisfaction derived from consuming a product or service. In the question above, Chan is deriving the same amount of satisfaction/utillity (25 utils) from the quantity of donuts and cheese that he buys. There is no reason to change the ratio in which he buys them.
If he was deriving more satisfaction from one of the two, we could say that he should buy more of that item to maximize his satisfaction. But this is not the case.